1. Technical Field
The present invention relates to a print head for scanning a light beam onto a scan target surface and an image forming device using the print head.
2. Related Art
As a light emitting element provided with an optical lens, which can be used for a print head, a light emitting element described, for example, in a document 1 (JP-A-2002-170662, paragraph 6-7, FIG. 8) has been proposed. The light emitting element described in the document 1 is formed on an opening section of a light guide hole, which is a through hole provided to a substrate, on one surface of the substrate. Further, the opening section thereof on the other surface of the substrate is provided with an imaging lens, as an optical lens, bonded thereto. Here, the light guide hole is a columnar hole perpendicular to both of the one surface and the other surface of the substrate. In a print head using the light emitting element described above, a light beam emitted from the light emitting element is guided to the imaging lens by the light guide hole, and focused by the imaging lens to form a spot on a scan target surface.
However, in such a print head as described above, since the light guide hole is the columnar hole perpendicular to both the one surface and the other surface of the substrate, the inside diameter of the light guide hole is constant in the thickness direction. Therefore, the opening section of the light guide hole on the other surface thereof abutting on the imaging lens as an optical lens functions as an aperture section defining the light beam input directly to the imaging lens. As described above, the aperture section is positioned nearer to the imaging lens than the front focal position of the imaging lens. Therefore, most of loci of principal rays respectively passing through the centers of the light beams passing through the opening section (the aperture section) of the light guide hole on the other surface thereof do not pass through the front focal point. Therefore, most of the loci of the principal rays emitted from the imaging lens fail to be substantially parallel to the optical axis of the imaging lens, and enter the scan target surface at various angles. Further, the loci of the light beams other than the principal rays converge respectively on the loci of the principal rays by passing through the imaging lens. According to these circumstances, a ratio in length between an adjacent spot distance and a constant spot pitch varies in conjunction with a variation in the distance between the imaging lens and the scan target surface. The adjacent spot distance is defined as a distance between two spots on the scan target surface corresponding respectively to one and the other of two adjacent light emitting elements the closest to each other belonging respectively two light emitting element groups having the smallest group pitch from each other. As a result, there arises a problem that a plurality of spots having the spot pitch and the adjacent spot distance different in length from each other is formed on the scan target surface. Further, in the image forming device using such a print head, there arises a problem that an undesired shading pattern is apt to be caused to degrade images.